With reference to FIGS. 7 to 9 illustrating a screen printing apparatus and FIG. 10 illustrating a flow chart of a screen printing method of the prior art, the screen printing method of the prior art is described.
As an apparatus for printing solder paste to a printed board 50 equivalent to the circuit board, there has been provided a screen printing apparatus 30 which comprises a recognition device 10 and a printing device 20, as shown in FIG. 7.
The recognition device 10 comprises a stage 1, motors 3 to 5 for moving the stage 1 in X, Y and .theta. directions as illustrated, a recognition camera 2 for recognizing recognition marks 51, 52 drawn on the printed board 50 as well as recognition marks 24, 25 drawn on a screen 21, and a control device 6 electrically connected to the recognition camera 2 and the motors 3 to 5.
The printing device 20 comprises the screen 21 on which a pattern showing the arrangement of openings for printing solder paste to the printed board 50 is formed, and a squeegee 22 for spreading the solder paste 23 placed on the screen 21 over the whole surface of the screen 21.
The control device 6 controls the drive of the motors 3 to 5 based on image information obtained from the recognition camera 2 in order that the pattern of the openings of the screen 21 matches a pattern printing position on the printed board 50 where the pattern should be printed on the printed board 50.
In this screen printing apparatus 30 having such a constitution, the stage 1 reciprocatingly moves between a printed-board loading/unloading position 182 and a just-under-the-screen position 183 where the stage 1 is located just under the screen 21, relative to the screen 21 that is fixed. Due to this movement, the printing operation is executed according to the steps as shown in FIG. 10.
More specifically, at Step (represented by "S" in the FIG.) 1, the printed board 50 is placed on the stage 1 at the printed-board loading/unloading position 182. At Steps 2 to 4, positional correction of the printed board 50 relative to the screen 21, which will be detailed later, is carried out. Thereafter, at Step 5, the stage 1 is moved to the just-under-the-screen position 183. Also at Step 5, the stage 1 is moved toward the screen 21 at the just-under-the-screen position 183. At Step 6, the screen 21 and the printed board 50 are brought into contact with each other, in which state printing is performed. In this printing process, the squeegee 22 lowers to the screen 21, and the squeegee 22 is moved leftward or rightward in the figure, by which the solder paste 23 is printed to the printed board 50 according to the pattern of the openings of the screen 21. After the printing, at Step 7, the stage 1 lowers, returning from the just-under-the-screen position 183 to the printed-board loading/unloading position 182 once again, whereby the printed board 50 after the printing process is removed from the stage 1. Then, another printed board 50 to be printed is placed on the stage 1 again, and the above operations are repeated.
The positional correction of the printed board 50 in Steps 2 to 4 is now explained.
The positional correction is to correct the position of the printed board 50 relative to the screen 21 so that the printed board 50 to be printed is placed at a specified position relative to the fixed screen 21. Information for achieving such positional correction is obtained by recognizing the recognition marks 24, 25 of the screen 21 and the recognition marks 51, 52 of the printed board 50, which is implemented by the recognition camera 2 moving. A detailed description is given below with reference to FIGS. 11 and 12.
First, the recognition of the recognition marks 24, 25 of the screen 21 is explained. A distance (SX1, SY1) from the origin 2a of the recognition camera 2 with respect to the recognition mark 24, and a distance (SX2, SY2) from the origin 2a of the recognition camera 2 with respect to the recognition mark 25 are calculated by the following equation [1]: EQU SX1=Sx1+Cx1.times.SCx1 EQU SY1=Sy1+Cy1.times.SCy1 EQU SX2=Sx2+Cx1.times.SCx2 EQU SY2=Sy2+Cy1.times.SCy2 [1]
where
Sx1, Sx2 are coordinate values of the recognition marks 24, 25 in the X direction, respectively, when the camera origin 2a is taken as the origin of the X-Y coordinate axes;
Sy1, Sy2 are coordinate values of the recognition marks 24, 25 in the Y direction, respectively, when the camera origin 2a is taken as the origin of the X-Y coordinate axes;
Cx1 is the resolution of the recognition camera 2 in the X direction, and Cyl is the resolution of the recognition camera 2 in the Y direction;
SCx1, SCx2 are the numbers of pixels in the X direction within the effective field of view with respect to the recognition marks 24, 25, respectively; and
SCy1, SCy2 are the numbers of pixels in the Y direction within the effective field of view with respect to the recognition marks 24, 25, respectively.
A distance from the camera origin 2a to the midpoint of the recognition marks 24, 25 is assumed as (SMX, SMY), and an internal angle formed by a straight line that connects these recognition marks 24, 25 to each other and a straight line parallel to the X direction is assumed as .theta.s. In addition, the above distance (SMX, SMY) and angle .theta.s represent the target position.
Next, a recognition mark 60 of the stage 1 is recognized in a state in which the stage 1 is located at the printed-board loading/unloading position 182 and in a state in which the stage 1 has been moved to the just-under-the-screen position 183, respectively, by which a travel amount (LX, LY) of the stage 1 is calculated.
After these operations are completed, the recognition of the recognition marks 51, 52 on the printed board 50 is performed. For this recognition operation, as in the foregoing case of the recognition marks 24, 25 of the screen 21, distances (PX1, PY1), (PX2, PY2) from the camera origin 2a to the recognition marks 51, 52 as shown in FIG. 12 are calculated by the following equation [2]: EQU PX1=Px1+Cx1.times.PCz1 EQU PY1=Py1+Cy1.times.PCy1 EQU PX2=Px2.times.Cx1.times.PCx2 EQU PY2=Py2+Cy1.times.PCy2 [2]
where
Px1, Px2 are coordinate values of the recognition marks 51, 52 in the X direction, respectively, when the camera origin 2a is taken as the origin of the X-Y coordinate axes;
Py1, Py2 are coordinate values of the recognition marks 51, 52 in the Y direction, respectively, when the camera origin 2a is taken as the origin of the X-Y coordinate axes;
PCx1, PCx2 are the numbers of pixels in the X direction within the effective field of view with respect to the recognition marks 51, 52, respectively; and
PCy1, PCy2 are the numbers of pixels in the Y direction within the effective field of view with respect to the recognition marks 24, 25, respectively.
A distance from the camera origin 2a to the midpoint of the recognition marks 51, 52 is assumed as (PMX, PMY), and an internal angle formed by a straight line that connects these recognition marks 51, 52 to each other and a straight line parallel to the X direction is assumed as .theta.p.
Based on these descriptions, a sequence of operations for recognition and correction of the printed board 50 are now explained. A new printed board 50-1 is carried in and placed on the stage 1, and the recognition camera 2 positions the printed board 50-1 to the aforementioned positions (PX1, PY1), (PX2, PY2) by a command of the control device 6. In the same way as with the recognition operations performed on the printed board 50, recognition marks 51-1, 52-1 of the printed board 50-1 are recognized by the recognition camera 2, respectively. As a result, midpoint (PMX-1, PMY-1) and angle .theta.p-1 between the two points are determined, respectively. Distances (PX1-1, PY1-1), (PX2-1, PY2-1) from the camera origin 2a to the recognition marks 51-1, 52-1, respectively, are calculated by the following equations: EQU PX1-1=Px1+Cx1.times.PCx1-1 EQU PY1-1=Py1+Cy1.times.PCy1-1 EQU PX2-1=Px2+Cx1.times.PCx2-1 EQU PY2-1=Py2+Cy1.times.PCy2-1
where
PCx1-1, PCx2-1 are numbers of pixels in the X direction within the effective field of view with respect to the recognition marks 51-1, 52-1, respectively; and
PCy1-1, PCy2-1 are the numbers of pixels in the Y direction within the effective field of view with respect to the recognition marks 51-1, 52-1, respectively.
Also, EQU PMX-1={(PX2-1)-(PX1-1)}/2 EQU PMY-1={(PY2-1)-(PY1-1)}/2
Adding the foregoing stage travel amount (LX, LY) to the midpoint (PMX-1, PMY-1) allows the position of the printed board 50-1 in the printing device 10 to be calculated.
Accordingly, compared with the case in which the printed board 50 is placed, move amounts .DELTA.x, .DELTA.y, .DELTA..theta. of the stage 1 with respect to the printed board 50-1 can be determined by the following equations: EQU .DELTA.x=SMX-{PPMX-1)+LY} EQU .DELTA.y=SMY-{(SMY-1)+LY} EQU .DELTA..theta.=.theta.s-{(.theta.p-1)}
Move of the stage 1 is controlled based on these movements (.DELTA.x, .DELTA.y, .DELTA..theta.), so that the stage 1 is moved to the printing device 20.
As described above, in the screen printing apparatus of the prior art, the printed board 50 is position-corrected by recognizing the recognition marks of the screen 21 and the printed board 50 then is transported to the screen 21, by which the printing is accomplished. As a result, the apparatus has had a disadvantage of being incapable of treating those printed boards 50 or the like having no recognition marks drawn thereon.
Further, in the screen printing apparatus of the prior art, as described above, each time a printed board 50 is placed on the stage 1, positional correction is performed for each printed board 50. However, the placement position where printed boards 50 are placed on the stage 1 would not differ among the individual printed boards 50 to such an extent that trouble occurs in printing. Accordingly, the prior-art screen printing apparatus was to execute unnecessary steps for each printed board, which would cause unnecessary increases in the time required for the printing of one printed board.